1. Field of the Invention
The invention relates to a junction box for dissipating heat from a solar panel.
2. Description of the Prior Art
According to prior art, solar installations incorporate solar panels of different sizes, whereby the size is determined by the number of series-connected solar cells. In large panels, furthermore, several rows of cells are connected in series. Each row of cells is bridged with a protecting diode installed in the reverse direction. In most cases, a Schottky diode is used, because the forward voltage drop is smaller than with a conventional silicon diode.
When operating solar cells, however, it may happen that individual solar cells are in the shade. The result is that the protecting diode becomes conducting, because the cathode has a lower electrical potential.
Without the protecting diode, the so-called Zener effect would produce a high power loss in the shaded solar cell, leading to a hot spot in this cell and, thus, almost always to destruction of the same. The solar panels, on the other hand, are for their part connected in series, so that a correspondingly high DC voltage passes with currents of up to 6 amperes to an inverter, which produces the AC voltage required for infeeds into the public grid.
The inverters are usually equipped with a communication module, in order to, for example, monitor the power output of the solar installation, either directly by way of a display or with the aid of a computer. The communication module also offers the possibility of remote polling, for example by telephone or via the Internet.
The Schottky diodes arranged above the rows of cells are very important components, because they bridge the respective row of cells when turned off and thus ensure that the current produced by the unshaded cells continues to flow, without the risk of destroying the solar cells. The voltage drop across the diode in the forward direction is relatively low at approx. 0.3 to 0.5 V.
Overtemperature and high field strengths, as could result from lightning strike, for example, lead to destruction of the diode. In the case of a fault, the diode can either open, i.e. there is an interruption between the anode and the cathode, or the diode can short out, in which anode and cathode melt together by arc-welding.
If a diode is short-circuited in operation, only the affected rows of cells are disabled; the system output drops, but this is practically negligible when several panels are in use. If the diode opens, however, this will initially have no effect on operation, provided the row of cells is producing sufficient power. If a cell is shaded, however, the diode can no longer conduct current, a hot spot then arises at the shaded cell, which may lead to destruction of that cell.
To date, only the power output of the whole solar installation is monitored, but not that of the individual panels. In case of a fault, i.e., when the power output of the installation drops, it is thus necessary to check the panels one by one until the defective path is found. Depending on the local circumstances, this is troublesome and time-consuming, and may even be dangerous, for example, when working on a roof. Consequently, repair for a large solar installation is usually complex and thus expensive.
The more critical case for the solar panels, namely a break in a protecting diode, is currently not monitored. As a result, such a failure of a protecting diode often leads to destruction of the cost-intensive solar cells.
In addition to monitoring of the functionality of the protecting diodes, it is important for reliable operation and a long service life of the solar panels to protect the protecting diodes from thermal overloads. When a solar cell is shaded, namely, the protecting diodes must handle high power levels, i.e. they become very hot. Inadequate cooling of the diodes, or even no cooling at all, can lead to immediate failure in extreme cases; in any case, excessive warming will significantly reduce the service life of the diodes. Nevertheless, it is still usual to operate the diodes without particular heat dissipation measures, whereby, in practice, exclusively plastic housings are used, which conduct the heat away only poorly.
In DE 100 50 614 C1, therefore, it is suggested that the protecting diodes be screwed to the inner side of a metal housing, which conducts the heat better; the outer sides of the metal housing can furthermore be provided with cooling fins. There is no teaching, however, as to how the diodes are to be insulated electrically from the housing. Because the cathode or anode of practically all diodes is always connected with the metal side of the diode packaging, the described solution would necessarily cause a short-circuit among the protecting diodes. Furthermore, the proposed screw connection of the diodes will loosen under the influence of temperature fluctuations and the resulting curvature of the metal side of the diodes, with the result that the thermal contact between housing and diode is interrupted. Moreover, the box cannot be opened, repaired or disassembled in case of defective components in the box, because it is adhesively sealed and the seal joint filled. It is also disadvantageous that the module construction is too high, and thus does not fit between the solar module and the roof in installations as they are mounted nowadays; furthermore, the connector channel is arranged for the axial direction, for which there is similarly no space. Given these disadvantages, the proposed solution remained state of the art on paper only.
In addition, DE 10 2004 036 697 A1 describes a junction box for panels of solar cells, which is characterized in that its base plate is made from a material with high thermal conductivity, and that an electrically insulating layer is provided between the base plate and the protecting diode. But here, too, the above-described problem, namely, the constant guarantee of a reliable contact between housing and diode, is not solved.